Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft either directly drives the generator rotor (“directly driven”) or through the use of a gearbox.
A variable speed wind turbine may typically be controlled by varying the generator torque and the pitch angle of the blades. As a result, aerodynamic torque, rotor speed and electrical power will vary.
A common prior art control strategy of a variable speed wind turbine is described with reference to FIG. 1. In FIG. 1, the operation of a typical variable speed wind turbine is illustrated in terms of the pitch angle (β), the electrical power generated (P), the generator torque (M) and the rotational velocity of the rotor (ω), as a function of the wind speed.
In a first operational range, from the cut-in wind speed to a first wind speed (e.g. approximately 5 or 6 m/s), the rotor may be controlled to rotate at a substantially constant speed that is just high enough to be able to accurately control it. The cut-in wind speed may be e.g. approximately 3 m/s.
In a second operational range, from the first wind speed (e.g. approximately 5 or 6 m/s) to a second wind speed (e.g. approximately 8.5 m/s), the objective is generally to maximize power output while maintaining the pitch angle of the blades constant so as to capture maximum energy. In order to achieve this objective, the generator torque and rotor speed may be varied so as keep the tip speed ratio λ (tangential velocity of the tip of the rotor blades divided by the prevailing wind speed) constant so as to maximize the power coefficient Cp.
In order to maximize power output and keep Cp constant at its maximum value, the rotor torque may be set in accordance with the following equation:T=k·ω2, whereink is a constant, and ω is the rotational speed of the generator. In a direct drive wind turbine, the generator speed substantially equals the rotor speed. In a wind turbine comprising a gearbox, normally, a substantially constant ratio exists between the rotor speed and the generator speed.
In a third operational range, which starts at reaching nominal rotor rotational speed and extends until reaching nominal power, the rotor speed may be kept constant, and the generator torque may be varied to such effect. In terms of wind speeds, this third operational range extends substantially from the second wind speed to the nominal wind speed e.g. from approximately 8.5 m/s to approximately 11 m/s.
In a fourth operational range, which may extend from the nominal wind speed to the cut-out wind speed (for example from approximately 11 m/s to 25 m/s), the blades may be rotated (“pitched”) to maintain the aerodynamic torque delivered by the rotor substantially constant. In practice, the pitch may be actuated such as to maintain the rotor speed substantially constant. At the cut-out wind speed, the wind turbine's operation is interrupted.
In the first, second and third operational ranges, i.e. at wind speeds below the nominal wind speed (the sub-nominal zone of operation), the blades are normally kept in a constant pitch position, namely the “below rated pitch position”. Said default pitch position may generally be close to a 0° pitch angle. The exact pitch angle in “below rated” conditions however depends on the complete design of the wind turbine.
The before described operation may be translated into a so-called power curve, such as the one shown in FIG. 1. Such a power curve may reflect the optimum operation of the wind turbine under steady-state conditions. However, in non-steady state (transient) conditions, the operation may not necessarily be optimum.
As further background, basic aerodynamic behaviour of (the blades of) a wind turbine is explained with reference to FIGS. 2a-2f. 
In FIG. 2a, a profile of a wind turbine blade is depicted in operation. The forces generated by the aerodynamic profile are determined by the wind that the profile “experiences”, the effective wind speed Ve. The effective wind speed is composed of the axial free stream wind speed Va and the tangential speed of the profile Vt. The tangential speed of the profile Vt is determined by the instantaneous rotor speed ωand the distance to the centre of rotation of the profile, the local radius r, i.e. Vt=ω·r.
The axial free stream wind speed Va is directly dependent on the wind speed Vw and on the speed of the wind downstream from the rotor Vdown, that is Va=½(Vw+Vdown). The axial free stream wind speed may e.g. be equal to approximately two thirds of the wind speed Vw.
The resultant wind flow, or effective wind speed Ve, generates lift L and drag D on the blade. A blade may theoretically be divided in an infinite number of blade sections, each blade section having its own local radius and its own local aerodynamic profile. For any given rotor speed, the tangential speed of each blade section will depend on its distance to the rotational axis of the hub (herein referred to as local radius).
The lift generated by a blade (section) depends on the effective wind speed Ve, and on the angle of attack of the blade (section) α, in accordance with the following formula:
      L    =                  1        2            ⁢              ρ        ·                  C          L                    ⁢                        V          e          2                ·        S              ,wherein
ρ is the air density, Ve is the effective wind speed, CL is the lift coefficient (wherein the lift coefficient is dependent on the angle of attack α) and S is the surface of the blade section.
Similarly, the drag D generated by a blade section can be determined in accordance with the following equation:
      D    =                  1        2            ⁢              ρ        ·                  C          D                    ⁢                        V          e          2                ·        S              ,wherein
CD is the drag coefficient dependent on angle of attack α.
For an entire wind turbine blade, the contribution to lift and drag of each blade section may be summed to arrive at the total drag and lift generated by the blade.
Both the drag coefficient CD and the lift coefficient CL depend on the profile or the blade section and vary as a function of the angle of attack of the blade section. The angle of attack α may be defined as the angle between the chord line of a profile (or blade section) and the vector of the effective wind flow, see also FIG. 2a. 
FIG. 2b illustrates in a very general manner how the lift coefficient and drag coefficient may vary as a function of the angle of attack of a blade section. Generally, the lift coefficient (reference sign 21) increases to a certain maximum at a so-called critical angle of attack 23. This critical angle of attack is also sometimes referred to as stall angle. The drag coefficient (reference sign 22) may generally be quite low and starts increasing in an important manner close to the critical angle of attack 23. This rapid change in aerodynamic behaviour of a profile or blade section is linked generally to the phenomenon that the aerodynamic flow around the profile (or blade section) is not able to follow the aerodynamic contour and the flow separates from the profile. The separation causes a wake of turbulent flow, which reduces the lift of a profile and increases the drag significantly.
The exact curves of the lift coefficient and drag coefficient may vary significantly in accordance with the aerodynamic profile chosen. However, in general, regardless of the aerodynamic profile chosen, a trend to increasing lift up until a critical angle of attack and also a rapid increase in drag after a critical angle of attack can be found.
In accordance with FIG. 2a, the tangential force generated by a blade section is given by T=L·sin(α+ϑ)−D·cos(α+ϑ), wherein ϑ is the pitch angle and α is the angle of attack. The pitch angle may be defined as the angle between the rotor plane and the chord line of a profile. Integrating the tangential force distribution over the radius provides the driving torque.
In order to increase the torque generated by the rotor, the angle of attack of any blade section is preferably kept below the critical angle of attack such that lift may be higher and drag may be lower.
It should be borne in mind that the angle of attack of each blade section depends on the tangential speed of the specific rotor blade section, the wind speed, the pitch angle and the local twist angle of the blade section. The local twist angle of a blade section may generally be considered constant, unless some kind of deformable blade is used. The tangential speed of the rotor blade section depends on the rotor speed (angular velocity of the rotor which is obviously the same for the whole blade and thus for each blade section) and on the distance of the blade section to the rotational axis.
For a given pitch angle, it follows that the angle of attack is determined by the tip speed ratio:
  λ  =                    ω        ·        R                    V        w              .  From this, it follows that the torque generated by a rotor blade section may become a rather complicated function of the instantaneous tip speed ratio and the pitch angle of the blade. This complicated relationship may be illustrated with a figure such as FIG. 2c. 
For every rotor blade section, the torque generated may be correlated to one of the lines of FIG. 2d of constant pitch angle. These lines depict the power coefficient (Cp), i.e. the ratio between the mechanical power captured by the wind turbine rotor and the available power in the wind, as a function of tip speed ratio λ and for different pitch angles. They may be obtained as cross-sections of planes of constant pitch angle with a figure such as the one shown in FIG. 2c. 
As the power captured by the wind turbine is directly related to the generated torque, Cp curves provide information about the torque dependence on pitch angle. For each pitch angle, there is a certain critical tip speed ratio. Below this tip speed ratio, stall may occur, i.e. the angle of attack is higher than the previously mentioned critical angle of attack.
This may be illustrated in an alternative manner, such as shown in FIG. 2e. For a given tip speed ratio, e.g. λ1, there is a certain critical pitch angle ϑcrit. This critical pitch angle gives the before-mentioned critical angle of attack for the given tip speed ratio. Below that critical pitch angle, stall may occur. At the same time for the given tip speed ratio, at the critical pitch angle, the captured power is maximum.
Now again with reference to FIG. 1, if the wind turbine is operating in the fourth operational range, i.e. in the supra-nominal zone of operation, the blades are pitched in an attempt to maintain the torque constant as the wind speed changes. In examples, this operational range may extend from the nominal wind speed to the cut-out wind speed.
It would be very hard to accurately adjust the pitch angle in response to wind speed measurements as obtained from a nacelle anemometer. A nacelle mounted anemometer will generally, due its location on top of the nacelle and behind the rotor, not measure the wind speed very accurately and its measurements may show a wind speed that largely varies with a high frequency. If the pitch system were to actuate on these measurements, it would constantly adjust the blade pitch (which would lead to premature wear of the pitch system) and the pitch system would not even be able to follow the commands that vary constantly. And if one also takes into account effects such as wind shear and wind veer, which cannot even be registered with a nacelle mounted anemometer, it becomes clear that the anemometer cannot be used for deriving pitch signals.
In practice therefore, instead of using measurements from an anemometer, the rotor speed is used. The rotor speed may be measured e.g. by measuring the generator rotor speed. In direct drive wind turbines, the rotor speed will correspond to the generator rotor speed, and in wind turbines employing a gearbox, there will generally be a fixed ratio between generator rotor speed and rotor speed.
The pitch system is then actuated in such a manner as to keep the rotor speed constant. This may work well in steady-state conditions or almost steady-state conditions, but in conditions which change relatively quickly, this may lead to undesirable results. This may be illustrated further with reference to FIG. 2e. 
Suppose a situation in which the wind turbine is operating in the supra-nominal zone with a tip speed ratio λ1. In this zone, the pitch angle will generally not be close to the critical pitch angle, wherein the critical pitch angle if the pitch angle corresponding for this particular situation to a critical angle of attack. The pitch angle will be higher than the critical angle, as the pitch of the blades is used to reduce the loads on the rotor and maintain aerodynamic torque substantially constant.
Let's now suppose that a sudden wind gust occurs, i.e. a significant increase in wind speed in a relatively short time. Due to the inertia of the rotor, the rotor speed will not immediately increase. As a consequence, also the pitch system will not immediately react to the increase in wind speed. It may be however, that due to the increase in wind speed, the wind turbine is now operating at another tip speed ratio, e.g. λ2. (because the wind speed changes, but the rotor speed has not changed).
At this other tip speed ratio, λ2, with the same pitch angle, stall may occur in the wind turbine blades, since the angle of attack of the blades may be above the critical angle of attack. With reference to FIG. 2e, the point of operation may have moved from point A to point B.
That is, in order to maintain an optimum angle of attack at the moment of the sudden wind gust, the pitch angle should be increased. Nevertheless, as the pitch system depends on the rotor inertia, it cannot track a sudden wind change, so blade pitch remains somewhat stuck, thus resulting in a large angle of attack. Depending on the precise effects of the wind, and the inertia of the rotor, it may be that the rotor speed even decreases a little bit, due to the separation of the flow from the blades. In response to this decrease in rotor speed, the pitch system will reduce the pitch angle more, thus aggravating the situation by further increasing the angle of attack.
The above situation may be particularly troublesome in case of e.g. a Mexican hat wind gust, such as the ones depicted in FIG. 2f. Mexican hat wind gusts are defined in the IEC 64100-1 2nd edition 1999-02 standard, since they may be particularly dangerous wind gusts. This standard defines Mexican hat wind gusts at various speeds, and at various azimuth angles.
The loads a wind turbine suffers during such a wind gust are severe and may define design loads for the wind turbine. This is due to the decrease in wind speed, before the high increase in wind speed (see FIG. 1). When the wind speed decreases, the pitch system tries to adapt the blades to this decrease (the blades are initially rotated in such a way to increase the aerodynamic torque by increasing their angle of attack, i.e. the pitch rate is below zero). With the pitch adaptation still ongoing, a significant increase in wind speed occurs. The aerodynamic torque and the thrust force on the hub can thus be very high. The pitch of the wind turbine will then start to adapt to these new wind conditions. However, the wind speed keeps increasing and due to the inertia of the system, the pitch can possibly not be adapted quickly enough, thus leading to the wind turbine potentially stalling and suffering increased loads. A typical pitch system may have an inherent pitch limitation of approximately 5°/second. Such a pitch rate may in principle be fast enough to respond to wind variations occurring during operation of the wind turbine. In general, the limiting factor may not be the pitch drive system but the means used to sense wind speed, i.e. rotor speed.
There still exists a need for a method of operating a wind turbine that at least partially reduces the aforementioned problems.